1. Field of Invention
This invention relates to an improved double wall tank having an outer tank of steel and an inner tank of thermoplastic material. More particularly, this invention has an outer tank with at least two fittings thereon and the inner tank is separate and apart from the outer tank except that it is bonded to outer tank in the area of the fittings.
2. Description of the Prior Art
It is known to have outer steel tanks with inner tanks made from thermoplastic material. The inner tank is formed by rotation molding within the outer tank and separates from the outer tank when the tank cools (see U.S. Pat. No. 4,625,892). When the inner tank cools, the thermoplastic material shrinks. In tanks that have two or more fittings, as the inner tank shrinks, that portion of the inner tank wall located between the two fittings is placed under enormous stress. The stress can cause the inner tank to rupture or to fail prematurely at the corners where the inner tank bends into the flanges for the fittings.
It is also known to line a tank with an inner layer of polymeric material where the polymeric material is bonded to an inner surface of the tank (see U.S. Pat. No. 5,358,682). An advantage of a double wall tank where there is a space between the inner tank and the outer tank is that a leak detection system can monitor the space between the two walls.